1. Field of the Invention
This invention relates generally to the field of compression release engine retarders for internal combustion engines. More particularly, it relates to a compression release engine retarder employing an hydraulic valve actuating mechanism having two master pistons for each slave piston wherein the master pistons are separately driven by pushtubes associated with the appropriate engine exhaust or intake valves or fuel injectors.
2. Prior Art
Engine retarders of the compression release type are well-known in the art. Such engine retarders are designed to convert, temporarily, an internal combustion engine of the spark ignition or compression ignition type into an air compressor so as to develop a retarding horsepower which may be a substantial portion of the operating horsepower developed by the engine.
As a general rule, so long as the retarding horsepower developed during retarding operations does not exceed the operating horsepower for which the engine was designed, the stresses on the crankshaft, bearings and drive train, though opposite in direction, will not exceed the allowable stresses for these parts and therefore the addition of the compression release retarder will not adversely affect the operating life of the drive train components of the engine and vehicle. At the same time, the engine retarder will supplement the braking capacity of the primary vehicle wheel braking system and extend, substantially, the life of the primary braking system of the vehicle. The basic design for an engine retarding system of the type here involved is disclosed in the Cummins U.S. Pat. No. 3,220,392.
The compression release engine retarder of the type disclosed in U.S. Pat. No. 3,220,392 employs an hydraulic system wherein the motion of a master piston controls the motion of a slave piston which, in turn, opens the exhaust valve of the internal combustion engine near the end of the compression stroke whereby the work done in compressing the intake air is not recovered during the expansion or "power" stroke, but, instead, is dissipated through the exhaust and radiator systems. The master piston is customarily driven by a pushtube controlled by the engine camshaft. It will be apparent that the force required to open the exhaust valve will be transmitted back through the hydraulic system to the pushtube and the camshaft. In order to minimize modification of the engine, it is common to utilize an existing pushtube which moves close to the desired time to operate the engine retarder hydraulic system. In some cases an exhaust valve pushtube associated with another engine cylinder is selected while, in other cases, it is convenient to use the fuel injector pushtube associated with the engine cylinder the exhaust valve for which is to be opened by the slave piston.
However, by assigning a second function to an existing pushtube, the possibility exists that an increased load may be experienced which, under some circumstances, may exceed the design capacity of the pushtube or camshaft. One approach to this problem is shown in the Sickler et al. U.S. Pat. No. 4,271,796 which discloses an automatic pressure relief system to unload the hydraulic system rapidly whenever an overpressure is sensed in the system, and then to reset the system. Another pressure relief system is disclosed in Egan U.S. Pat. No 4,150,640. A pressure unloading system responding to excess motion of the slave piston is disclosed in Laas U.S. Pat. No. 3,405,699. While the pressure unloading systems of the prior art are effective to prevent damage to the valve train and other components of the engine, they necessarily reduce, at least temporarily, the retarding horsepower of the engine.
A disadvantage associated with the use of an existing pushtube to control the hydraulic system of a compression release retarder also resides in the fact that the timing of the pushtube motion is designed for its primary function in the engine and may not be optimum for the compression release retarding function. One approach to this latter problem is disclosed in application Ser. No. 248,344, now U.S. Pat. No. 4,398,510, assigned to the assignee of the present application, which provides a timing advance mechanism whereby the normal clearance or "lash" necessarily incorporated into the valve mechanism to provide for dimensional changes resulting from temperature variation is decreased or eliminated during the compression release retarding operation. The effect of decreasing or eliminating "lash" is to advance the timing of the compression release retarding event so that it approaches an optimum timing. This mechanism, however does not modify the stress on the pushtubes except to the degree that the force required to open an exhaust valve varies with the timing of the compression release event.